With all the bar bags I've made (all sizes, diameters and weights of material) and 1 harness, I like to get them as close to the bike as possible using a daisy chain to loop a strap around the head tube. I find attaching to the bars and then to the head tube creates a very stable, jostle free load and personally looks better/tighter (yes I care about looks
For aero bars you have your handlebars to attach the roll to and then the aero extensions as an additional attachment point which could help stabilize the load. I've never done this but have rode with with aero bars with risers and a second stem. Again I prefer the bag tighter and closer to the bike and attached to the head tube to minimize flopping.
Be interesting to hear others findings as to the stability of the load especially with the bar roll less full and potentially floppy.